One thing that I like about Common Lisp and Ruby is the ability to examine class definitions, etc. at runtime. I have several model classes in a Common Lisp application and I want to implement a web services API for my application that in some cases returns model objects to the requester.
I wrote a very short utility function today that takes any class instance looks up all of the slot names, and then gets the slot values. I output to an XML stream.
I don't have to manually change any code to marshall any of my classes to XML if I change the model class definitions.